Determination of pH of a Solution

Determination of pH of a Solution

The change of potential with change in concentration of ions has been used in chemistry in many ways. Like all other electrodes, the potential of a hydrogen electrode which is reversible with respect to hydrogen ions changes with change in the concentration of H⁺ ions in accordance with the Nernst equation. Based on this fact methods have been devised to determine the pH of a solution from measurements of potentials of electrochemical cells. For this purpose a cell is set up with a standard electrode and an electrode containing the solution whose pH is to be measured. The standard electrode used for these measurements is usually a calomel electrode. The cell can be written as-

Pt│H₂ (1 atm) (c_H 1.0 molL^-1)││Cl^– (1.0 molL^-1)│Hg₂Cl₂(s)│Hg

Using the Nernst equation the e.m.f of the cell at 25°C can be expressed as-

E_cell = E_B – E_L = E_calomel – E_H+

E_cell = E_calomel –{E_H+ + 0.0591/2 log [H⁺]²}

As at the hydrogen electrode the reaction taking place is-

2H⁺ + e ↔ H₂

but, E⁰_H+ = 0.00 V

Hence, E_cell = E_calomel – 0.0591 log [H⁺]² = E_calomel + 0.0591 pH

or, (E_cell – E_calomel) = 0.0591 pH

Therefore, pH = (E_cell – E_calomel) / 0.0591